1. Field of the Invention
This invention relates to a process for producing high-strength, high-porosity structures for use as components of molten carbonate fuel cells, having good in-cell performing properties without the use of expensive alloys and/or alloying elements. In particular, this invention relates to a process for producing porous structures, such as electrodes and bubble barriers in molten carbonate fuel cells using low cost metal powders and/or metal oxide powders and ceramic oxide powders.
2. Description of Prior Art
Porous structures suitable for use in molten carbonate fuel cells made from metal alloys are well known in the prior art. Common among such structures are structures made from nickel/aluminum alloys, nickel/chromium alloys, and nickel/copper alloys. U.S. Pat. No. 4,999,155 teaches a method for forming porous oxide dispersion strengthened molten carbonate fuel cell anodes in which an alloy comprising a base metal and an alloying metal is formed, preferably in powder form, and sintered to form a porous anode structure in a reducing environment. The resulting structure is subsequently subjected to conditions in which the base metal is reduced and the alloying metal is oxidized, thereby causing internal oxidization of the alloying metal. Sintering is carried out at a temperature in the range of 500.degree. C. to 1000.degree. C. in an inert atmosphere comprised of nitrogen, argon, helium, or alternatively, in a vacuum or gas-tight container. The base metal is selected from the group consisting of nickel, copper, and cobalt. The alloying metal is selected from the group consisting of iron, chromium, aluminum, titanium, silicon, beryllium, magnesium, thorium, and yttrium. U.S. Pat. No. 5,041,159 similarly teaches a nickel/aluminum alloy which is simultaneously oxidized and sintered at temperatures of 800.degree.-1100.degree. C. in an atmosphere in which the alloying material (aluminum) is exclusively substantially internally oxidized forming a sintered porous plaque containing nickel metal and oxidized alloying material. U.S. Pat. No. 4,762,558 teaches a process for producing nickel aluminide, Ni.sub.3 Al, in which a mixture of elemental nickel powder and elemental aluminum powder are sintered in a vacuum or dry inert atmosphere at a temperature of 500.degree.-750.degree. C. U.S. Pat. No. 4,939,111 teaches a porous sintered cathode for a molten carbonate fuel cell formed from a metal oxide and lithium manganate or lithium ferrite at a temperature up to 950.degree. C.
The preparation of sintered structures also is taught by U.S. Pat. No. 4,996,022 in which a process for producing a sintered body in which one or more metal powder particles are mixed with an organic binder and formed into a predetermined shape is disclosed. After removing the binder from the "green" body by preheating the "green" body in an inert gas atmosphere at a temperature up to about 800.degree. C., the resulting porous body is sintered at a temperature up to about 1200.degree. C. U.S. Pat. No. 4,992,233 similarly teaches a process for sintering alloys of metal powders, preferably aluminum and iron powders, into structures in which the metal powders are prefired at a temperature of 500.degree.-650.degree. C. in an oxidizing atmosphere followed by sintering in an inert or reducing atmosphere at a temperature of about 1275.degree.-1450.degree. C. U.S. Pat. No. 4,654,195 teaches a method for fabricating ribbed electrodes by depositing a composition of nickel, copper, and mixtures thereof and chromium onto a ribbed mold, pre-firing the molded composition in a reducing atmosphere at temperatures of 650.degree.-900.degree. C. and subsequently sintering the resulting electrode in a reducing atmosphere at temperatures of 1050.degree.-1450.degree. C. And, U.S. Pat. No. 4,410,607 teaches a process for preparing a porous sintered lithiated nickel oxide plaque in which a finely divided particulate oxide material is blended with a binder to form a solid mass which is subsequently sintered at a temperature of about 1000.degree.-1200.degree. C.
U.S. Pat. No. 4,411,968 teaches a composite matrix material for molten carbonate fuel cells having a matrix tape portion and a bubble barrier portion, the matrix tape portion comprising a sheet of a mixture of ceramic particles, particles inert to the molten carbonate environment and an organic polymer binder which burns off or volatilizes under fuel cell operating conditions. The bubble barrier portion is a fine pore, ion permeable sheet of material which is bonded to the matrix tape.
U.S. Pat. No. 4,407,775 teaches a process for producing structures from metallic (alloy) powders, such as high speed steel alloy powders and certain high alloy ferrous powders, in which the powder is formed into the desired shape and sintered at a temperature of about 1200.degree.-1250.degree. C. in an inert atmosphere or a vacuum.
U.S. Pat. No. 4,386,040 teaches a process for producing a lithiated nickel oxide (lithium doped nickel oxide) cathode for a molten carbonate fuel cell in which a "green" body consisting of a mixture of nickel powder and lithium oxide is fired in air at temperatures of about 600.degree.-1000.degree. C. producing a fired body of Li.sub.x Ni.sub.(1-x) O and having a porosity of at least about 50% by volume of the total volume of the body.
U.S. Pat. No. 4,251,344 teaches an electrode having a porous nickel surface formed from a paste consisting of a powder mixture of NiAl.sub.3 and Ni.sub.3 B which is pre-fired at a temperature of about 450.degree.-650.degree. C. in air and subsequently sintered in a nitrogen atmosphere at a temperature of about 800.degree.-900.degree. C. followed by removal of the intermetallic aluminum, boron and boron oxide by dissolution with sodium hydroxide, forming the electrode. U.S. Pat. No. 4,985,071 teaches a process for producing a base metal thin film for use as an electrode using a printing technique in which a thin layer of a base metal oxide is formed on a substrate and sintered at temperatures of 600.degree. C. and below in a reducing atmosphere. U.S. Pat. No. 4,943,496 teaches a process for making a fuel cell electrode in which a metal powder of nickel, cobalt, copper, chromium and iron, and mixtures or alloys thereof, is mixed with an organic binder and water to form a paste, dried at room temperature and fired in a hydrogen atmosphere at a temperature of about 750.degree.-1000.degree. C. After impregnating one surface with a water repellent solution, the other surface is treated with a ceramic oxide precursor solution and then, the electrode is fired in a reducing atmosphere. This process is carried out in a reducing atmosphere and as such does not utilize a reactive oxide method which is carried out in air or an oxygen containing gas. U.S. Pat. No. 4,902,587 teaches an anode for a molten carbonate fuel cell prepared from base materials of metal and metal oxides including nickel, chromium and cobalt and chromium oxide in which the mixture of base metals is sintered, with chromium being selectively oxidized.
U.S. Pat. No. 4,708,917 and related U.S. Pat. No. 4,800,052 teach a process for producing molten carbonate fuel cell cathodes from metal oxides including nickel powders which are pre-fired at temperatures of 600.degree.-1000.degree. C. in an oxidizing atmosphere, formed into the desired electrode shape, and sintered at a temperature of 850.degree.-1250.degree. C. in an oxidizing atmosphere. U.S. Pat. No. 2,837,428 teaches a process for sintering chromium-alumina metal ceramics in which the powders are formed into a desired shape and sintered at a temperature of about 1500.degree.-1900.degree. C. in an inert, reducing, or non-oxidizing atmosphere, or in a vacuum. U.S. Pat. No. 3,607,433 teaches a method for preparing electrodes from metals or metal oxides in which the starting material is connected to a porous surface of an electrolyte and heated at temperatures from 1190.degree. C. to 1800.degree. C., depending on the oxide used as a starting material, in an inert atmosphere. The resulting electrode is cooled below 1000.degree. C. and reduced in a furnace or in-situ in a fuel cell. U.S. Pat. No. 3,821,036 teaches a process for strengthening metals and alloys in which a base metal powder containing oxygen is blended with a reactive metal powder, pressed, sintered, densified and held below the sintering temperature of about 1500.degree. -2400.degree. C. to form an ultra-fine dispersion reactive metal oxide throughout the base metal. U.S. Pat. No. 3,050,386, U.S. Pat. No. 3,953,237 and U.S. Pat. No. 4,436,794 generally teach electrodes having a base material of metal and/or metal oxides.
The use of alloyed metals for producing components of molten carbonate fuel cells is also taught by U.S. Pat. No. 4,238,557 which teaches sintering of nickel and chrome molded articles; U.S. Pat. No. 4,247,604 which utilizes nickel or cobalt alloyed with up to 20% chromium; and U.S. Pat. No. 4,659,379 which utilizes nickel/aluminum alloys to produce molten carbonate fuel cell anodes by first partially oxidizing the powder so that the exterior of the material is a nickel oxide layer and the interior of the material is a nickel metal throughout which is dispersed the oxide of the alloying material.
Other prior art references teaching methods for preparing molten carbonate fuel cell components are U.S. Pat. No. 4,317,866 which utilizes a ceria anode; U.S. Pat. No. 4,386,960 which utilizes a nickel or copper coated ceramic powder encapsulated by a metal component; U.S. Pat. No. 4,797,379 which adds an alkali hydroxide or alkali hydroxide mixed with a ceramic constituent to a preexisting nickel, nickel-cobalt, or nickel-chromium porous structure; and U.S. Pat. No. 4,752,500 which teaches deposition of a stabilizing agent into a preexisting structure of nickel, cobalt, copper and mixture and alloys thereof by dipping the structure into precursor aqueous solutions of the desired agent.
A major disadvantage of known methods of preparing porous metallic structures having ceramic oxides dispersion strengthening agents inside the metal grain is their use of expensive metal alloy powders made of a base metal and an alloying metal element or mixtures of metal powders made of a base metal and an alloying metal element. If alloys are used, the oxide of the alloying element if formed prior to, during or after sintering; and, if mixtures of metal powders are used, they are sintered first to form an alloy and then the alloying element is oxidized. In either case, the cost for preparing porous metallic structures using metal alloys is substantial due to the high cost of metal alloys.